1. Field of the Invention
The present invention generally involves magnetic resonance tomography, or MRT, as applied in medicine for examining patients. The present invention relates especially to a process for improving flow measurements as they are performed in magnetic resonance tomography, for example, to show vascular systems that have blood flowing through them.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been used successfully as an imaging modality for over 15 years in medicine and biophysics. In this examination method, an object is exposed to a strong, constant magnetic field. In the process, the nuclear spins of the atoms in the object, which were previously randomly oriented, become aligned. Radio-frequency energy then can excite these “aligned” nuclear spins into a certain oscillation. This oscillation generates the actual MRT measurement signal, which is acquired using suitable receiver coils. By using non-homogenous magnetic fields, generated by gradient coils, signals from the measurement object can be spatially coded in all three spatial directions, which, in general, are called “spatial coding”.
The recording of data in MRT is done in k-space (frequency domain). The MRT-image in the image domain is linked using Fourier transformation to the MRT-data in k-space. The spatial coding of the object that spans k-space is done using the aforementioned gradients in all three spatial directions. In the process, a distinction is made between the slice or layer (specifies the recorded slice in the object, usually the z-axis), the frequency coding (specifies a direction in the slice, usually the x-axis) and the phase coding (determines the second dimension within the slice, usually the y-axis). Furthermore, by phase coding along the z-axis, the selected slice can be subdivided into sub-slices.
Thus, initially a slice is selectively excited, for example, in the z-direction, and phase-coding is possibly performed in the z-direction. The coding of the spatial information in the slice is done through a combined phase and frequency coding using the two aforementioned orthogonal gradient fields, which, in the example of a slice excited in the z-direction, are generated by the aforementioned gradient fields in the x- and y-directions.
A possible form for recording the data in an MRT measurement is shown in FIGS. 4A and 4B. The sequence applied is a spin-echo sequence. In this sequence, the magnetization of the spins is made in the x-y plane by a 90° excitation pulse. In the course of time (½ TE; TE is the echo time), a dephasing of the magnetization portions, which together form the transverse magnetization in the x-y plane Mxy, occurs. After a certain time (e.g. ½ TE), a 180° pulse is emitted in the x-y plane such that the dephased magnetization components are reflected without changing the precession direction and precession speed of the individual magnetization portions. After an additional time period ½ TE, the magnetization components point in the same direction again, i.e. a regeneration of the traverse magnetization results (called “rephasing”). The complete regeneration of the transverse magnetization is called spin-echo.
In order to measure a corporate slice of the object to be examined, the imaging sequence is repeated N-times for different values of the phase coding gradients, e.g. GY. The time interval of the respective excitation producing HF-pulses is called the repetition time TR. The magnetic resonance signal (spin-echo signal) is also scanned, digitized, and stored, in the presence of the read-out gradients GX, N-times at equivalent time intervals Δt in each sequence pass by the Δt-clocked ADC (Analog Digital Converter). In this way, according to FIG. 4B, a numerical matrix that is created line-by-line (matrix in the k-space or k-matrix) with N×N data points. From this dataset, using a Fourier transformation, a MR-image of the slice in question can be directly reconstructed with a resolution of N×N pixels (a symmetrical matrix with N×N points is only one example, asymmetric matrices also can be generated).
For speed-indicating flow measurements in magnetic resonance tomography, either the progression of the average speed of the flowing medium in a certain vessel can be determined during a movement cycle (breathing, heart movement) or the speed distribution in the cross-section of the vessel region that is of interest and in which a fluid is flowing through can be determined at a defined point in time of the movement. Of great interest, for example, is the speed progression (curve) of the blood in the aorta during a cardiac cycle (from systole to systole).
For such measurements the imaging slice typically oriented vertically to the vessels to be displayed, and an additional phase coding gradient is generated in the direction of the flowing medium (blood, secretions, etc.). The additional (phase-coding) gradient in the flow direction is necessary to be able to assign a defined speed to each voxel of the flowing medium on the basis of the intensity of the resonance signal of the nuclear spin it contains. This allocation normally takes place after the actual measurement in a software-supported post-processing by the user on the screen. Prior to the actual speed resolution measurement an overview image (localizer) is acquired. The user manually marks the region of interest ROI in the localizer in the slice (for example using a mouse) and begins the measurement of a series of images (typically 20 images per cycle), whereby the marked ROI is correspondingly propagated and reproduced by means of segmenting algorithms.
After the measurement of the image series the users begins an intensity analysis in the marked region by means of which either the speed is determined over the entire ROI of each image of the series, or a speed profile (curve) is created in the form of a gray-scale distribution of each image of the series.
The flow in the recorded images can now be displayed by means of time sequence, e.g. at a frequency of 20 images per second, on the screen as a movie.
The speed-dependency of the nuclear resonance signal of the flowing material is based on the different distribution of saturated as well as completely relaxed or unrelaxed spins of the perfused vessel of the layer to be displayed. To obtain an optimum resolution of the speed or of the speed distribution in the vessel, the present speed interval (velocity encoding, VENC) of the flowing material should be exactly known, in order to be able to perform an optimized speed coding by means of the speed coding gradients to be generated created dependent on the layer thickness, the flip angle and further measuring or sequence parameters.
In the present state of the art the user (in general the physician) has to estimate the flow speed of, for example, blood in a vessel to be measured. The flow speed varies quite greatly dependent on the anatomical position. For example the maximum speed of the blood in the aorta differs greatly from that in the carotids or in the stenotic vessel regions. The user therefore sets empirical values which—to cover in any event the entire speed region—as a rule define too large an interval. The result is a speed resolution tat is below optimum.